1. Field of the Art
The present invention relates to a novel holographic recording material composition capable of recording a volume phase hologram.
2. Description of the Related Art
A hologram is known as a pattern of refractive index or absorptance recorded on a photosensitive material, and has been used in a broad range of optical elements, a stereographic display, interference measurement and recording or processing of images or information as it has multiple functions.
As conventional and representative material compositions for holographic recording, gelatin dichromate photosensitive material or silver salt photosensitive material subjected to bleaching process has been used. Although these materials have high diffraction efficiencies, there are disadvantages that they require complicated processes for producing a hologram, in particular a wet development process.
As a dry photosensitive material for hologram by which the above-mentioned disadvantages are overcome, the products of OmniDex (trademark; manufactured by E.I. du Pont de Nemours, Inc.) are commercially available. The material contains a radical polymerization monomer and a binder polymer, a photo-radical polymerization initiator and a sensitizing pigment as main components, and records a hologram by utilizing the difference of refractive indices between the radical polymerization monomer and the binder polymer. Specifically, when the photosensitive material formed in a shape of film is subjected to interference exposure, radical polymerization is initiated at an area where light is strongly irradiated, and in accordance with it, a concentration gradient of the radical polymerization monomer is made, and the concentration gradient causes the diffusion of the radical polymerization monomer from areas where light is weakly irradiated to areas where light is strongly irradiated. As a result of it, low and high portions in density of radical polymerization monomer and density of polymerized polymer are made, and a hologram is formed as a variation of the refractive indices among the above-mentioned monomer, polymer and binder polymer. Although the material system has the best performance as a photopolymer for hologram reported at present, applications are limited to a thickness of the order of 20 μm, and it is pointed out that the material has problems in heat resistance and transparency.
U.S. Pat. No. 5,453,340 or U.S. Pat. No. 5,665,494 (these related patent: JP-A-5-107999) discloses a material system in which radical polymerization and cationic polymerization are simultaneously used. U.S. Pat. No. 5,759,721 discloses a material system in which cationic polymerization used. However, these materials are composed of only organic materials, and thus problems on mechanical strength or environmental stability have not fully been elucidated yet.
In addition, U.S. Pat. No. 6,479,193 or U.S. Pat. No. 6,524,771 (these related patents: JP-A-6-019040 and JP-A-6-148880) discloses a material system in which an inorganic material network and a photopolymerizable monomer are simultaneously used. In a case where an inorganic material capable of forming a network is used as a binder, there are advantages that it is excellent in heat resistance, environmental resistance and mechanical strength, and it can enlarge the difference of refractive indices between the binder and the photopolymerizable organic monomer. However, the hologram recording film formed from the material system has problems that it is relatively brittle, and is not good in flexibility, processability or coating performance, and that it is difficult to prepare a homogeneous coating material as the compatibility of the inorganic binder with the organic monomer is not good.
Further, U.S. Pat. No. 6,236,493 (related patent: JP-A-2000-508783) discloses a material in which super fine metal particles are dispersed into a solid matrix. However, the invention disclosed therein has problems that it is requested to confer fluidity upon the matrix and is not good in solid performance.
In addition, U.S. patent application Ser. No. 2002/0110740 (related patent JP-A-2002-236440) discloses a holographic recording material in which an organic-inorganic hybrid polymer and organic metal fine particles having photopolymerization reactive groups are used. However, the invention disclosed therein has a problem as an industrial process as it requires heating and UV polymerization for fixing interference fringes.
As a material for making hologram recording in a simpler manner, JP-A-2003-84651 or N. Suzuki, Y. Tomita, and T. Kojima (Appl. Phys. Lett., 2002, Vol. 81, p. 4121-4123) discloses a holographic recording material in which inorganic fine particles are dispersed into photopolymerizable monomers. However, the invention disclosed therein has a problem that it has a large light scattering as the fine particles have a large particle diameter and broad particle size distribution.